Plate 44 Lip A Mucocutaneous Junction

The lips are the entry point of the alimentary canal. Here, the thin keratinized epidermis of face skin changes to the thick parakeratinized epithelium of the oral mucosa. The transition zone, the red portion of the lips, is characterized by deep penetration of connective tissue papillae into the base of the stratified squamous keratinized epithelium. The blood vessels and nerve endings in these papillae are responsible for both the color and the exquisite touch sensitivity of the lips.

A H&E-stained sagittal section through the lip in this low-power orientation photomicrograph to the right (X8) reveals the skin of the face, the red margin of the lip, and the transition to the oral mucosa. The numbered rectangles indicate representative areas of each of these sites, shown at higher magnifications in Figures 1, 3, and 5, respectively, on the adjacent plate. Note the change in thickness of the epithelium from the exterior or facial portion of the lip (the vertical surface on the right) to the interior surface of the oral cavity (the surface beginning with rectangle 5 and continuing down the left surface of the lip) in this micrograph.

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